A decanter centrifuge generally comprises an imperforate bowl mounted for rotation about its central longitudinal axis. The bowl typically includes a cylindrical section and a frusto-conical section at one end. A screw conveyor is coaxially mounted within the bowl and adapted for rotation at a differential speed with respect to the bowl. The screw conveyor typically comprises a central hub having a series of conveyor flights extending radially therefrom and forming a helix along the length of the hub.
The rotation of the bowl of the decanter centrifuge creates a centrifugal force which separates a liquid feed mixture or slurry into its constituent parts. The feed mixture within the bowl forms a cylindrical pond, with a ring or layer of the heavy constituent material(s) adjacent the inside bowl wall and a ring or layer of the lighter constituent material(s) radially inward of the heavy material layer.
The terms "heavy phase" and "light phase" are often employed to describe the materials which are separable from the feed mixture by the application of centrifugal force within a centrifuge. In a centrifuge having a conveyor, the light phase material will usually be a liquid and the heavy phase material will usually be a mixture of solids, also including some liquid. The liquid feed mixture or slurry introduced into the bowl has a specific concentration of suspended solids or other insoluble material therein. These solids are concentrated by the centrifugal force to form the heavy phase mixture within the bowl, including coarse solids, fine solids and liquid. Because of the variations in density of the solids as well as the varying effect of the centrifugal force acting on the feed within the bowl, the concentration of the separated heavy phase (expressed as a percent solids) varies at different positions within the centrifuge bowl. The concentration of the heavy materials that do not settle or separate from the light phase material also varies (expressed as milligrams per liter). The term "interface" is often employed to define the dividing line between the heavy and light phase layers formed within the bowl. The location of the interface within the bowl will vary depending upon the operational parameters of the centrifuge, the axial position within the bowl and the qualities of the feed mixture. In describing the operation of the centrifuge, the interface is often visualized as a sharp dividing line. However, as presently understood, the interface in a typical liquid/solids-type separation of a decanter centrifuge is in the form of a concentration gradient or transition zone of indeterminate thickness.
The discharge of the heavy phase material from the bowl of a decanter centrifuge is accomplished as a function of the differential rotation of the conveyor with respect to the bowl. The differential speed causes the conveyor flights to move the heavy phase material along the inside bowl wall toward the tapered end of the bowl. A discharge path is provided at the restricted end of the bowl, with the conveyor flights moving the heavy phase over a weir surface. The clarified light phase material typically flows in an opposite direction from that of the heavy phase. A light phase discharge path is provided in the cylindrical end of the bowl, with the liquid also flowing over weir surfaces. The intent of a decanter centrifuge is to continuously and separately discharge the heavy phase and light phase constituent parts of the feed mixture.
One form of a decanter centrifuge is shown in Brautigam U.S. Pat No. 3,764,062. The cylindrical hub of the centrifuge conveyor includes a central hollow portion having a series of openings positioned at various locations around the periphery of the hub. A feed tube introduces the feed into the hub. The feed mixture is discharged through the openings directly into the bowl. The Brautigam patent is herein incorporated by reference.
Lavanchy U.S. Pat. No. 4,245,777 shows a variation of the Brautigam decanter centrifuge. The Lavanchy centrifuge includes a feed cone within the bowl, projecting from the periphery of the conveyor hub. The feed cone directs the feed material from the openings in the conveyor hub into the bowl. The conical surface of the feed cone includes a series of accelerator veins thereon for directing the feed liquid down the surface of the cone. This Lavanchy patent is also incorporated herein by reference.
Lee U.S. Pat. No. 3,795,361 shows in one embodiment a decanter centrifuge having a conical feed cone within the bowl. The Lee feed cone projects radially outward within the bowl, through the interface and into the heavy phase/solids layer. An alternate structure shown in Lee includes a baffle in the form of an annular disc. The Lee cone and disc type baffles assist in creating a centrifugal pressure head within the bowl to assist the discharge of the heavy phase from the bowl. This centrifugal pressure head is also created by positioning the weir surfaces of the light phase discharge path radially inward of the position of the heavy phase weir surfaces (known as the "spillover" position). This Lee patent is also incorporated herein by reference.
The projection of the annular cone or radial disc within the Lee-type decanter centrifuge forms a separating zone for the feed mixture between the baffle and the liquid discharge end of the bowl. On the opposite side of the baffle is created a discharge zone for the heavy phase. Only the heavy phase passes under the radial periphery of the baffle due to the seal formed with the heavy phase layer. Because of the creation of a seal at the radial periphery of the baffle, a pressure imbalance may be created within the separating zone of the bowl to provide a discharge force through the restricted passageway (formed by the baffle and the inside bowl wall) for the discharge of heavy phase from the conical bowl portion through the heavy phase discharge path.
A loss of a seal between the baffle and the heavy phase layer in the Lee-type decanter centrifuge creates a condition called "washout". A washout is a sudden reduction in the solids concentration being discharged from the centrifuge due to the underflow of both heavy and light phase through the restricted passage and into the discharge zone. This washout is typically visualized by the interface in the separating zone moving close to or beyond the radial periphery of the baffle, allowing the centrifugal pressure head to drive feed material into the heavy phase discharge end and out of the heavy phase discharge path from the bowl.
A typical application for a Lee-type decanter centrifuge is in a thickening operation. Thickening is generally defined as discharging a heavy phase cake which is less than 10% solids. Usually the appearance of a thickened heavy phase is that of a viscous pudding. In certain thickening applications, difficult-to-convey materials can only be discharged from a decanter centrifuge by a Lee-type construction. A dewatering-type operation, differing from thickening, generally includes a level of dryness in the discharged heavy phase that is greater than a 10% concentration. The viscosity of the normal dewatered heavy phase is typically much greater than that of the thickening-type operation. In some dewatering applications, the Lee-type construction is not required.
Typically, the performance of a decanter centrifuge, including the Lee-type centrifuge, improves with an increase in the length of the separating zone and/or an increase in the rotational speed of the bowl. Modern materials and equipment have permitted greater rotational speeds that result in an increase in the "G" level acting to separate the feed mixture within the bowl. However, the length of the bowl is typically limited by the natural frequency of the conveyor as positioned on its bearings. The natural frequency must be higher than the maximum operating speed in order to avoid destructive vibration. As a result of this physical relationship, typically, decanter centrifuges have included large diameter conveyor hubs so as to provide the necessary transverse and torsional stiffness.
One way of increasing the length of the separation zone in the bowl is to increase the angle of the frusto-conical portion of the bowl, i.e., increasing the angle between the beach and the axis of rotation. A deeper pond is desirable because it increases the residence time of the feed and, thus, improves capacity. Deeper ponds obtained by reducing the radius of the pond surface also result in lower power demand by the centrifuge. This reduced power demand is proportional to the square of the discharge radius for the clarified liquid and solids. As an example, the reduction of the pond radius by twenty percent will result in a 44% reduction of power demanded by the centrifuge. This modification also reduces turbulence in the feed portion of the separation zone.
The rate of feed into the centrifuge is also a determining factor in the success of the overall separation operation. Not only will the fixed rate affect the time for which a mixture is subject to centrifugal force, but such may also cause turbulence that remixes already separated heavy phase/solids. For example, the openings in the conveyor hub in the Brautigam patent (discussed above) at high rates creates jets causing turbulence within the feed portion of the separating zone. If this type feed structure is positioned adjacent a Lee-type baffle, secondary flows may also be created, resulting in relatively high velocities at the interface. Turbulence and secondary flows near the interface make stability difficult and a washout more likely. This may be particularly true where the viscosity of the heavy phase decreases as the flow velocity increases, making loss of seal more likely.